Manufacture of sodium tripolyphosphate



April 15, 1947.

c. s.. KING MANUFACTURE OF SODIUM TRIPOLYPHOSPHATE Filed Sept. 14, 1942 Gabo Patented pr. 15, 1947 MANUFACTURE OF SODIUM TRIPOLY- PHOSPHATE Charles S. King,l Joliet, Ill.,- assigner to Blockson Chemical Co., Joliet, Ill., a corporation of Illinois Application September 14, 1942, Serial No. 458,201

(c1. zs-los) 18 Claims. 1

The present invention relates generally to the production of sodium polyphosphates, and in particular to an anhydrous thermal process for producing sodium tripolyphosphate in substantially pure form with substantially 100% yield, and to a modification whereby, as desired, a predeter-l mined content of tetrasodium pyrophosphate may accompany the sodium tripolyphosphate.

'I'he soduim polyphosphates are considered as those salts having compositions lying in the system represented by NaPOa to Na4P2Or, or in words, by sodium metaphosphate to tetrasodium pyrophosphate.

The extreme formulas defining the range are such that each of the extremes may be considered as dehydrated known salts. Thus, NaPOs may be considered as a dehydratedy form of NaHzPOi; and Na4P2O1 may be considered as a dehydrated form of NaiHPOi. In fact these respective monosodium orthophosphate and disodium orthophosphate salts may be dehydrated by thermal action to produce sodium metaphosphate (NaPOs) and tetrasodium pyrophosphate Within the polyphosphate system sodium tripolyphosphate has the formula NasPaOio. .it does not correspond to any dehydrated known salt, and therefore, it is not known that it may be made by a comparable dehydrating reaction. However, it is known that it may be formed by a thermal reaction between two known compounds, as by the reaction:

Since the factors of the above equation are de-` hydrated salts as stated above, the reaction may -be given as:

It has -been known that said salts in the proportion of Reaction 2 may be heated to dryness, then thermally treated at more elevated temperature to form to some extent only a limited quantity of NasPsOio, unless very precise controls of tempering are practiced in which case excellent yields may be obtained. (SeeU. S. Patent No. 2,174,614.)

Sodium tripolyphosphate at elevated temperatures has characteristic reactions or behavior. It is a crystalline anhydrous product with two physical forms in the solid state below its decomposition temperature of about 625 C. There is a critical or transition temperature of about 500 C. below which a form stable from about 500 C.

2 lowv about 500 C., and above which the latter form may be tempered back to the form which is stable from about 500 to 625 C.

At the temperature of about 625 C. sodium tripolyphosphate begins to decompose with the formation of a liquid phase (which is a eutectic mixture) and. a solid phase (which is NaiPzO'z). At about 860 C., the solid phase disappears and a homogeneous melt appears. 'I'hese reactions are all reversible in nature.

In cooling a homogeneous fusion mass corresponding in composition to the formula NasPaOm, but not necessarily being such, it is possible to derive various compositions which are mixtures of polyphosphates. There is a constant shift in composition, and by controlled cooling, then chilling, it is possible to obtain a high yield of NasPaOiu Such a process is costly in that high temperature must be attained in order to secure the homogeneous liquid fusion from which to start, and

high temperature must be retained in the slow' cooling process.

In attempts toavoid this, as by proceeding with materials and in proportion, as in Reaction 2 above, evaporating, and heating, but not to a fusion point, only small yields of Nat-,P3010 have been attained, and it has been contaminated with both NaPOa and NaiPzO'z, these being unreacted or reversion products of dehydration of the said factors of Reaction 2. The NaPOa may be Apresent as a Water insoluble form, and in large amount.

It ls the object of the present invention to produce sodium tripolyphosphate in high yield, substantially free from any form of NaPOa, and if desired, substantially free from Na4P2O'1 or containing the latter in predetermined amount.

Various other and ancillary objects and advantages of the invention will become apparent from the following description and explanation of the invention which is given in connection with the accompanying drawings in which:

Fig. 1 is a vertical longitudinal section of a cylindrical rotary furnace, in more or less diagrammatic representation. 1

Fig. 2 is a vertical cross-section of the furnace of Fig. 1, taken on the line 2-2 thereof.

Since diiiiculties have been experienced heretofore in processes aiming'at the production of NasPsOio by thermal reactions short of a temperature producing fusion, the reaction has been studied and the underlying cause of the low yield to 625 C. may be tempered to a form stable be- 55 and the resulting impurities has been discovered,

3 and remedied` It has been discovered that th physical state of aggregation in the evanescent mass undergoing thermal treatment is most important. The main reaction involves a redistribution of the oxides Na20 and P205 in the proportion of 5Na20 and 3P2O5. When the reacting oxides for -a-theoretical 100% yield are supplied according to Reactions 1 and 2 given above, as a result of which there are two groups, one having a ratio of Na20 to P205 of 1 to l, and the other having a ratio of NazO to P205 of 2 to 1, the latter group must yield some of its Na20 or must receive P205, or both actions occur together.

According to the present invention the integral bodies of these two groups are formed s as to have exceedingly minute size and to be so well mixed, that the effect of concentrating Na20 in one group, and of concentrating P205 in the other group, thereby isolating each from reaction, is substantially avoided. It has as yet been impossible to ascertain whether the particle sizes are microscopically visible, sub-microscopic, or substantially molecular in size. However, by the methods employed to produce the mixture, the neness is such that the effects of coarseness of the mixture are avoided, these being low yield, and undesired NaP or undesired Na4P201. The physical condition is expressed herein by the term finely mixed. The condition may vary from intimate admixture to homogeneity,

According to the present invention a mixture of reacting salts or chemicals to provide Na20 and P205 in the desired proportion, in solution in waterfis sprayed, or fed in a iine stream, or otherwise dispersed in iine liquid particles or small volumes, into or onto a very hot medium sucient to ash-dry the solution and produce finely mixed ash or solid residue, preferablygin ne particle sizes. The chemicals used are such as to provide only Na20 and P205 in the residue, in the desired proportion. In the solution other components of the chemicals employed may exist, but they must be such asto be volatilized away and not deposited in the residual solid salt mass, which latter ultimately consists analytically of from at least 5 to less than 6 moles of Na20 and 3 moles of P205. These components other than NaaO and P205 may be H20, C02 or NHs, or radicals such as oxalates, citrates and like volatile or combustible" radicals. The solid particles of residue maintain their integrity and internal distribution of Na20 and P205 in the proportion supplied by the liquid. Even though the particles may ball up or agglomerate, this is true. The solid residue, agglomerated or not, is then roasted at a suitable temperature below a fusion point of the mass at any time in the process, whereby the oxides combine in substantially 100% yield to form polyphosphate salts. Where the ratio of Na20 to P205 in the ash is 5 to 3, the yield is substantially 100% sodium tripolyphosphate. Where the proportion is 6 to 3, the end product would be Na4P2O7, but this is outside and at the limit of the present invention, since merely dehydration of Na2HP04 would produce it without interaction of zziolecules. Therefore, where the ratio of Na20 to P205 in the ash is at least 5 and less than 6 moles of Na20 to 3 moles of P205, the present invention may be practiced to secure a substantially 100% yield of NasPsOio with a ratio of 5 to 3, and a predetermined mixture of Na5P301o and Na4P201 otherwise, in each case, without any substantial quantity of any form of NaPO3.

In carrying out the invention a strong solution of mixed salts or other chemicals is provided such as to give only NazO and P205 in solid salt residue, and to give the desired ratio of NazO to P205.

The agents selected for dissolution may be two or more in number, and chosen in kind and quantity to produce the desired reactive salt mass which is infusible in the process, so as to effect reaction in the solid state. Where only two agents are used there must be one selected from a. group of agents, herein designated group A, in which P205 exceeds Na20, if present, by more than 3 moles of P205 to 5 moles of NazO; and the other agent must be selected from a group of agents, herein designated group B, in which NazO exceeds P205, if present, by more than 5 moles of Na20 to 3 moles of P205. The following listings show suitable water-soluble salts or agents for such groups:

However, it is to be understood that the invention is not limited to selection from groups A and B,.nor are groups A and B limited to lists shown above. Extension of the lists, in view of the teachings herein made, will be obvious to those skilled in the art. It will also be obvious that extensions of the lists of available material, as

to the combustible organic acid salts such as so- It is preferred that they be provided as set forth in Reaction 2 from the commonplace orthophosphate salts, to be dehydrated in process. Such a solution is preferred with a strength at 50 B.

The desired solution is preferably used as a saturated one, to limit the amount of water to be evaporated. The strength of the solution will depend upon the selection of salts and the temperature at which it is held.

The solution may be sprayed upon a hot plate of 550 C., for example, where a small batch, as in a laboratory, is to be made. But in commercial operation a continuously operating furnace may be provided for the process. A suitable one is shown in the drawings.

Numeral I0 represents an inclined cylindrical rotary furnace with a slightly higher entry end II than discharge end I2. 0i1 or gas burners I3 on fuel line I4 at the entry end project hot flames downwardly toward the bottom interior. A solution feed line I5 parallels the fuel line and has nozzles I6, which introduce either a fine continuous stream, or which direct a mist of the solution, into the hot iiamc, thereby flash-drying it, and depositing particles of nely mixed solid residue to build up a bed of it designated I1.

An annular partition I9 divides the 'furnace into the ilame section and the heating section, whereby a depth of bed I1 is created. Surplus material of the bed, as agglomerated particles, spills over the partition as the furnace is rotated and the material progresses through the roasting section 20 of the furnace as bed 2| to discharge end I2. A controlled lcurrent of air is passed through the furnace, as shown by arrows 22. The air stream is adjusted with respect to the burners in operation, to maintain the bed I1 at about 550 to 600 C., and to discharge material 2| and air at the end l2 at about 350 C. About one hour through the furnace suiiices.

Under the conditions of the roasting process, where a preferred temperature in the range from 300 to 350 C. is used, any isolated monosodium orthophosphate of which the bed-forming content would remain isolated, would dehydrate to insoluble sodium metaphosphate. However, in the present invention there is no isolated monosodium orthophosphate of which the bed-forming content remains isolated. Such bed-forming content is intimately associated with other bed-forming content from material of the class of group B. The roasting effects reaction between the 'bed-forming content from group A and the bed-forming content from group B, thus preventing the vformation of insoluble sodium metaphosphate from any monosodium orthophosphate or its equivalent which may be initially employed.

These temperatures assure dehydration or reaction, or both, in the preferred usage. After dehydration to-effect an intimate mixture equivalent to at least one mole of NaiPzOv to one mole of NaPOa, whatever the salts that are present, the reaction takes place slowly but appreciably where the roasting temperature is about 250 C., and it is fast Where the roasting temperature is 300 C. However, it is preferred to use a roasting temperature of at least 300 C. but rather from 325 to 350 C. to insure complete and rapid reaction in the largest agglomerates. The temperature may be as high as 600 C., but there is no advantage economically.

The temperature where the solution is introduced is not critical for reaction purposes, but is such, depending upon the size and amount of spray particles being introduced, to eiect a substantial flash-drying of the particles, whereby to avoid depositing liquid from which slow evaporation and fractional crystallization could occur.

Reference is made to my copending application Serial No. 549,747, led August 16, v1944, as a continuation-in-part of the present application, wherein a distinctive result is achieved by a temperature control in a limited range within the broad range herein set forth and claimed.

Numerous changes and modifications in the tion in water of salt having a content of NazO and of P205 in the ratio of 1Na20 to 1P205, and other salt having a content-of Na20 and P205 in the ratio of 2Na20 to 1P205, there being suf,- cient of said salts together providing at least 5 and less than 6 molecular weights of Na20 and 3 molecular weights of P205, said solution having a composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of Na2O and P205 in the said proportion of said solution, nely dividing the solution, flash-drying the finely divided solution so as to provide said solid salt residue in nely mixed form, and heating said residue at a temperature in the range from 250 to 600 C., whereby to form a crystalline salt mass containing units corresponding to NasPaOio from substantially all of the first-mentioned salt.

3. The method which comprises forming a solution in water of solute consisting of monosodium orthophosphate and of disodium orthophosphate, there being sufficient of each of the said two salts to provide at least 5 and less than 6 molecular weights of Na20 to each 3 molecular Weights of P205; finely dividing the solution, ash-drying the nely divided solution so as to provide a solid salt residue in finely mixed form which is infusible at temperatures up to at least 600 C., and heating said residue at a temperature in the range from 250 to 600 C., whereby to form a crystalline salt mass containing units corresponding to Na5Pa01o from substantially all of the first-mentioned salt.

4. The method which comprises forming a water solution of material providing at least 5 moles and less than 6 moles of Na20 and 3 moles of P`2Os and of a composition to provide the hereinaiter-mentioned solid salt residue infusible at temperatures upto at least 600 C. which residue ultimately consists analytically of Na20 and P205 in the said proportion of said solution, flashdrying the solution by directing said solution in iinely divided form through a name-carrying atmosphere toward a heated bed of said solid salt residue so as to provide solid salt residue in nely mixed form as an addition to said bed, and thereafter heating the newly added material ofsaid bed at a temperature in the range from 250 to 600 C., whereby to effectthe reaction 'of the constituents thereof to a crystalline salt mass containing units corresponding to Na5P20io.

5. The method which comprises forming a so.. lution in water of at least two materials together providing Na20 and P205 in the proportion of at least 5 and less than 6 moles of Na20 to 3 moles of P205 of which materials at least one contains in excess'of 3 moles of P205 relative to a content of from 0 to 5 moles of Na20, said solution being of a composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of Na20 and P205 in the said proportion of said solution,` continuously directing said solution in finely divided form through a heated atmosphere toward a bed of such residue and thereby flash-drying the particles of the .solution so as to provide said solid salt residue in finely mixed form and so as to add the resulting solid salt residue to said bed, said bed having a temperature in the range from 250 to 600 C., and continuously moving the material in said bed away from said depositing residue for discharge of material substantially at the rate of providing residue, and maintaining the temperature of said bed material below about 600 C.

6. The method which comprises forming a solution of a mixture of dissolved substances which mixture is capable oi. providing the hereinaftermentioned solid salt residue infusible at temperatures up to at least 600 C. which residue u1- timately consists analytically o1 5moles of NazO and 3 moles of P205, continuously directing said solution in finely divided form through a heated atmosphere toward a bed of such residue and thereby dash-drying the particles of the solution so as to provide said solid salt residue in finely mixed form and so as to add the resulting solid salt residue to the said bed, said bed having a temperature in the range from 250 to 600 C., and continuously moving the material of said bed away from said depositing residue for discharge of material substantially at the rate of providing residue, and maintaining the temperature of said material below about 600 C.

'7. The method which comprises forming a solution of a mixture of dissolved substances which mixture is capable of providing the hereinaftermentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of at least 5 moles and less than 6 moles of Na20 and 3 moles of P205, continuously directing said solution in finely divided form through a heated atmosphere toward a bed of suc'n residue and thereby iiashdrying the particles of the solution so as to provide said solid salt residue in finely mixed form and so as to add the resulting solid salt residue to said bed, said bed having a temperature in the range from 250 to 600 C., and continuously moving the material of said bed away from said depositing residue for discharge of the material substantially at the rate of providing residue, and maintaining the temperature of said material below about 600 C.

8. In the process of forming sodium polyphosphate at temperatures not over 600 C., the steps which comprise forming a solution in water ofa mixture of dissolved substances including at least one compound of Na20 and at least one diierent compound of P205 which mixture is capable of providing the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of 5 moles of Na2O and 3 moles of P205, and dash-drying said solution so as to provide said solid salt residue in finely mixed pable of providing the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C., continuously directing said solution in nely divided form through a heated atmosphere toward a bed of such residue and thereby flash-drying the particles of the solution so as to provide Vsaid solid salt residue in finely mixed form and so as to add the resulting solid residue to said bed, said bed having a temperature inthe range from 250 to 600 C., and continuously mo' .g the material in said bed away -from'said depositing residue for discharge of material substantially at the rate of providing residue, and maintaining' the temperature of said bed of material below 600 C.

l0. The method which comprises forming a solution in water of material providing 5 moles of Na20 and 3 moles of P205 and of a composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of 5 moles of NazO and 3 moles of P205, finely dividing the solution, flash-drying the nely divided solution so as to provide said solid salt residue in nely mixed form, and heating the residue at a. temperature in the range from 250 to 600 C., whereby to forma crystalline salt mass consisting of units corresponding to NasPaOio whichmass is substantially free from NaPOs.

11. The method which comprises forming a solution in water of salt having a, content of Na20 and of P205 in the ratio of lNazO to 1P20s, and other salt having a content of Na20 and P205 in the ratio of 2Na20 and 1P205, said salts together providing 5 moles of Na20 and 3 moles of P205, said solution having a composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists of 5 moles of Na20 and 3 moles of P205, iinely dividing the solution, flash-drying the finely divided solution so as to provide said solid salt residue in finely mixed form, and heating the residue at a temperature in the range from 250 to 600I C., whereby to form a crystalline salt mass consisting of units corresponding to NasPaOio which mass is substantially free from sodium metaphosphate.

12. The method which comprises forming a solution in water of solute consisting of monosodium orthopliosphate and of disodium orthophosphate, there being sufficient of each of the said two salts to provide 5 moles of Na20 to 3 moles of P205; finely dividing the solution, flash-drying the finely divided solution so as to provide solid salt residue in finely mixed form which is infusible at temperatures up to at least 600 C., and heating said residue at a temperature in the range form, and heating said residue at a temperature there being sufficient of each of said two salts to provide at least 5 and less than 6 moles of NazO to each 3 moles of P205, said solution being cafrom 250 to 600 C., whereby to form a crystalline salt mass consisting of units corresponding to NasPsOio which mass is substantially free from sodium metaphosphate.

13. The method which comprises forming a solution in water of at let two materials together providing 5 moles of Na20 and 3 moles of P205 of which materials at least one contains in excess of 3 moles of P205 relative to a. content of from 0 to 5 moles of Na20, said solution being of a composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of 5 moles of NazO and 3 moles of P205, finely dividing the solution; dash-drying the finely divided solution so as to provide said solid salt residue in finely mixed form, and heating the said residue at a. temperature in the range from 250 to 600?J C., whereby to form a. crystalline salt mass consisting of units corresponding to Nal-,P3010 which mass is substantially free from NaPOa.

14. The method which comprises forming a water solution of material providing 5 moles of NazO and 3 moles of P205 and of a. composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of 5 moles of Na20 and 3 moles of P205, flash-drying said solution by directing said solution in inely divided form through a, flame-carrying atmosphere toward a heated bed of said solid residue so as to provide solid salt residue in finely mixed form as an addition to said bed, and thereafter heating the newly added material of said bed at a temperature in the range from 250 to 600 C., whereby to effect reaction of the constituents thereof to a crystalline salt mass consisting of units corresponding to Na5P5010.

15. The method which comprises forming a solution in water of at least two materials together providing 5 moles of NazO and 3 moles of P205 of which materials at least one contains in excess of 3 moles of P205 relative to a content of from to 5 moles of NazO, said solution being of a composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists KA fanalytically of moles of Na20 and 3 moles of P205, continuously directing said solution in finely divided form through'a heated atmosphere toward a bed of such residue and thereby nashdrying the particles of the solution so as to provide said solid salt residue in finely mixed form and so as to add the resulting solid residue to said bed, said bed having a temperature in the range from 250 to 600 C., and continuously moving the material away from said depositing residue for discharge of material substantially at the rate of providing said residue, and maintaining the temperature of said bed material below about 600 C.

16. In the process of forming sodium polyphosphate at temperatures not over 600 C., the steps which comprise forming a solution in water of a mixture of dissolved substances including at least one compound of Na20 and at least one different 'compound of P205 which mixture is capable of providing the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of at least 5 moles and less than 6 moles of Na20 and 3 moles o P205, and flashdrying said solution so as to provide said solid salt residue in finely mixed form, and heating said residue at a temperature in the range from 250 to 600 C., whereby the residue contents react to form sodium triphosphate.

17. The method which comprises forming a solution in water of solute consisting of monosodium orthophosphate and of disodium orthoph'osphate, there being suiiicient of each of said two salts to provide 5 moles of Na2O and 3 moles of P205, said solution being capable of providing the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C., continuously directing said solution in finely divided form through a heated atmosphere toward a bed of such residue and thereby flash-drying the particles of the solution so as to provide said solid salt residue in nely mixed -form and so as to add the resulting solid salt residue to said bed, said bed having a temperature in the range from 250 to 600 C., and continuously moving the material in said bed away from said depositing residue for discharge of material substantially at the rate of providing residue, and maintaining th`e temperature of said bed of material below 600 C.

18. The method which comprises forming a solution in water of at least two materials together providing Na20 and P205 in the proportion of at least 5 and less than 6 moles of NaaO to 3 moles of P205 of which materials at least one contains in excess of 3 moles of P205 relative to a content of from 0 to 5 moles of Na20, said solution being of a composition to provide the hereinafter-mentioned solid salt residue infusible at temperatures up to at least 600 C. which residue ultimately consists analytically of NazO and P205 in the said proportion of said solution, finely dividing the solution; Hash-drying the finely divided solution so as to provide said solid salt residue in finely mixed form, and heating the said residue at a temperature in th'e range from 250 to 600 C., whereby to form a crystalline salt mass containing units corresponding to NasPaOm which mass is substantially free from NaP and containing Na4P207 substantially in the amount necessitated by the content of Na20 in excess of said 5 moles.

CHARLES S. KING.

REFERENCES TED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,182,357 Schwartz Dec. 5. 1939 2,046,841 Preisman July 7, 1936 1,689,697 Thornton Oct. 30, 1928 2,174,614 Bornemann Oct. 3, 1939 FOREIGN PATENTS Number Country Date 441,474 British Jan. 20, 1936 649,757 Germany Aug. 19. 1937 

